I can suggest a script, but it would help if you described exactly how you set up the current format. For example, what do you want to use for the currency character (if any), the thousands separator, and the decimal separator.

Currency is Euro ( € ). Thousands separator is the point ( . ) and decimal separator is the comma ( , )

Example: € 1.234,56

Even a simple direction will do. I'm not new to JS but I never used it into PDFs, as I mainly use it in webpages to manipulate the DOM and such.I'm sure the solution is simpler than I think, but it's just confusing me at the moment

I just tossed the following together quickly and didn't do much testing, to take is as general guidance. It is intended to be placed in a document-level JavaScript and called from the field's custom Format script. You can't simply place it all as the Format script due to the way Acrobat behaves when you do.

The point of the script is to calculate the number to use as the first parameter to the AFNumberFormant function, which is what Acrobat uses when you set up a numeric format. I couldn't think of a clever name for the function, so I'll leave that to you:

function fmt1() {
// Set the minimum number of digits to the right of the decimal
var min = 2;
// Get the field value, as a string
var val = event.value;
// Round to the nearest 10-thousand of a cent
var rn = util.printf("%.4f", val);
// Replace any trailing zeroes with nothing
rn = rn.replace(/[0]+$/, "");
// Get the number of characters after the decimal
var num = rn.split(".")[1].length;
// Set to the minimum if appropriate
if (num < 2) num = 2;
// Use built-in formatting function
AFNumber_Format(num, 2, 0, 0, "\u20ac", true);
}

Call it like this in the field's Format event:

fmt1();

If this is not a calculated field, but one that a user will interact with, you'll want to add a corresponding Keystroke function, something like: